METHOD OF MAKING REACTION BONDED/HOT PRESSED SI.SUB.3N.SUB.4 FOR USE AS A CUTTING TOOL

摘要

A method of making a reaction bonded/hot pressed silicon nitride comprising object is disclosed. To form a protective amorphous glass on crystallites in the final product, an Al2O3/SiO2 ratio must be maintained which is greater than 2. Second phase crystallites are formed prior to hot pressing. a mixture of silicon, SiO2, and .4-2.3 molar percent (by weight of the silicon) of oxygen carrying agents, i.e., Y2O3 and Al2O3, is preformed and reaction nitrided to form discs or billets having at least 60% alpha Si3N4 and a high proportion of second phase crystallites which displace substantially all silicate glass except for a controlled small quantity (.2-1% by weight). The reactive amounts of Y2O3, Al2O3 and SiO2 are controlled to assure formation of substantially YlSi12O24N4 as the second phase crystallite. Al2O3 is controlled in an amount of .4-4% by weight to insure that the small proportion of glass has little or no microporosity associated therewith and serves to protect the oxynitrides against linear oxidation kinetics.